POWER DEVICES
Bi-directional power module for 48V mild hybrid electric vehicles
EPC explores a GaN-based design of a 2kW 48V/12V bi-directional power module for 48V mild hybrid electric vehicles
W
ith the increase in government mandates to combat climate change, automakers are moving quickly to leverage new technology to respond by switching from the internal combustion engine to electric-drive vehicles. The hybrid vehicle market has more than doubled from 2017 from 2.0% to 5.1% and by 2025, one in every 10 vehicles sold worldwide is projected to be a 48V mild hybrid. 48V systems boost fuel efficiency, deliver four times the power without increasing engine size, and reduce carbon-dioxide emissions without increasing system costs. These systems will require a 48V – 12V bidirectional converter, with power ranging from 1.5kW to 6kW. The design priorities for these systems are size, cost, and high reliability. GaN-based designs provide a solution to address all these priorities. Here, we will show the design of a 2kW, two-phase 48V/12V bi- directional converter using GaN FETs that achieves 96% efficiency and are targeted for the 48V mild hybrid system. A scalable solution; two converters can be paralleled for 4kW, three converters for 6kW or only one phase can be used for 1kW. A simplified schematic of the bi- directional DC-DC converter is shown in Figure 1.
48V/12V Bi-directional DC-DC converter design This design uses the EPC2302 GaN FET, which has a low inductance 3 x 5mm QFN package with exposed top for excellent thermal management. With 1.8mΩ RON, the rated peak dc current is 101A. Therefore, a two-phase approach is selected so that the FET current requirement is reduced. As an example, at 14V 2kW output, the dc current in each phase is 70A. This also reduces the current rating requirement for the inductors.
The MPQ1918-AEC1 gate drivers in this design are AEC-Q100 qualified and use a bootstrap technique with voltage clamping for driving the high side FET. These drivers also have fast propagation times and excellent propagation delay matching of less than 1.5ns typical. Vishay IHTH-1125KZ-5A series inductors offer high current ratings for the inductance. In this design, the 1.0µH inductor and 500kHz switching frequency is selected, resulting in 80A peak inductor current. To ensure accurate phase current balancing, current sensing using a precision shunt resistor is preferred over inductor DCR current sensing. The MCP6C02 current sense amplifier used in this design has a maximum bandwidth of 500kHz and 50V/V gain. This results in 10mV/A total current sensing gain for 0.2mΩ shunt.
Symmetrical layout between the two phases is also critical in phase current balancing and minimising other effects from mismatch, such as gate drive delay, switching transition speed, overshoot, etc.
Digital control
A dsPIC33CK256MP503 digital controller from Microchip is used in this design. It is a 16-bit processor with a maximum CPU speed of 100 MIPS. The pulse- width modulation (PWM) module can be configured in high-resolution mode, resulting in 250 ps resolution in duty cycle and dead times, allowing accurate adjustment of dead times to fully exploit the fast-switching capability of the GaN FETs. Digital average current mode control is implemented for both buck and boost modes. The current sensing circuitry consists of sense
28 DECEMBER/JANUARY 2023 | ELECTRONICS TODAY Figure 2: Digital average current mode control diagram
resistors and differential amplifiers. In this design, low loss 0.2mΩ sense resistors and low-noise amplifiers MCP6C02 are used. The control block diagram is shown in Figure 2. The same current reference IREF is used for the two independent current loops. As a result, the current in both inductors will be regulated to the same value. The bandwidth of the two inner current loops are set to 6kHz, and the outer voltage loop
Figure 1: Simplified schematic diagram of the bi-directional converter
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